Lubricating coatings are thin film coatings that increase the lifetime of a substrate by reducing damage caused by wear. Such solid coatings are required in applications where components are subjected to harsh environmental conditions such as high temperatures and intense radiation. Lubricating coatings find utility in precision applications which include but are not limited to machine tools (e.g. drill bits, end mills, etc.), close tolerance gimbals, bearings, shafts, and gears. Additionally, lubricating coatings are applied to wood surfaces which are subjected to friction. The use of lubricating coatings reduce the amount of friction between moving parts resulting in decreased heat production, stress, surface deformation and fatigue forces.
Many methods exist for depositing solid coatings onto a substrate. For example, coatings have been rubbed, bonded, thermally sprayed, and laser deposited onto substrates. Additionally, sputter deposition has been commonly used to coat substrates.
By utilizing any of these methods, film quality is limited and environmental concerns exist. For example, sputter deposition leads to contamination by background gases. Contamination can compromise film integrity and limit control of the film properties. Additionally, these methods typically require the use of organic solvents that contain a significant amount of volatile organic compounds (VOCs). These VOCs escape into the atmosphere while the composition dries. Such solvent based systems are undesirable because of the hazards and expenses associated with VOCs. The hazards include water and air pollution and the expenses include the cost of complying with strict government regulation on solvent emission levels.
In contrast, UV curable compositions contain reactive monomers instead of solvents; thus eliminating the detrimental effects of VOCs. UV compositions are cured through rapid photo-induced polymerizations without the use of solvents which emit VOCs. Since the UV curing process is essentially solvent free, the necessity for time consuming and expensive pollution abatement procedures is greatly reduced.
Additionally, UV curable lubricating coatings offer several other benefits. First, faster cure times offer substantial economic benefits. Furthermore, heat sensitive materials can be safely coated and cured with UV light without thermal degradation of heat sensitive substrates. Finally, UV light is a relatively low cost source of energy due to its widespread availability.
Although UV curable lubricant coatings are superior to thermally cured coatings, there are still disadvantages inherent in UV curable coatings. Since UV curable coatings require compositions which have high molecular weight and viscosity, it is difficult to apply the composition through spraying and brushing. Additionally, many UV curable lubricant coatings require compositions that are prone to dispersion and instability. By using these compositions, the curing results in an uneven and blemished coating.
Accordingly, there exists a need to provide environmentally safe UV curable lubricant coatings with enhance physical properties for use on substrates subjected to harsh environmental conditions. Additionally, there is a need to provide a method of applying the coatings which furthers the goal of improved performance.